Purification of polyolefins



United States Batent C) F 3,287,343 PURIFICATION OF POLYOLEFINS AbrahamKutner, Newark, Del., assignor to Hercules Incorporated, a corporationof Delaware No Drawing. Filed Dec. 10, 1963, Ser. No. 329,392 5 Claims.(Cl. 26093.7)

The present invention relates to a process for the purification ofstereoregular polyolefins prepared by a low-pressure process in a liquiddiluent.

There is known a process of polymerizing ethylene and other l-olefinsunder relatively mild conditions of temperature and pressure by using asa catalyst for the polymerization a compound of a metal of Groups IVB orV-B of the Periodic Table in combination with an organometallic compoundof a metal of Group III-A of the Periodic Table. The process is usuallycarried out by adding the catalyst to an inert organic diluent that isliquid under the reaction conditions and passing the ethylene or otherolefin into the catalyst mixture at atmospheric or slightly elevatedpressure and at room temperature or slightly above room temperature.When an olefin is so polymerized, a highly crystalline stereoregularpolymer is obtainable that has many industrial uses. In the process, thepolymer which is insoluble in the reaction medium precipitates out andcan be separated from the diluent by any of the usual means such asfiltration, centrifugation, etc. However, when no steps are taken topurify the polymer, it normally contains a large quantity of catalystresidues which adversely affect its color, its stability, and itselectrical properties, and render the polymer corrosive to metal. Hence,it has been necessary to devise methods for purifying these polymers torid them from the catalyst residues that are inherently present at thecompletion of the polymerization process.

Numerous methods for purifying the stereoregular polyolefins have beensuggested. One such method comprises washing the polymer, afterseparation from the polymerization diluent, with mineral acids as, forexample, methanolic hydrochloric acid, aqueous solutions of nitric acid,etc. This type of treatment gives very pure polymers but requires theuse of considerable quantities of expensive reagents.

By another process that has been suggested, the polyolefin, while stillslurried in the polymerization diluent, is treated with an alkanol tosolubilize the catalyst residues and the alkanol-containing slurry isthen washed with an aqueous liquid to extract the catalyst residues fromthe slurry. The diluent phase, containing the polymer, and the aqueousphase, containing the catalyst residues, are then separated bydecantation. The latter method generally gives excellent results in thatthe amount of metal residues contained in the purified polymer isgenerally reduced to about 200 parts per million. There is still need,however, for a process for purifying these polyolefins that reduces theamount of catalyst residues in the polymers to an even lower amount.

One of the most effective processes known for removing the metalliccatalyst residues from these polyolefin slurries is described in BritishPatent 840,233. In this process, the polymer slurry is treated with analcohol to solubilize the catalyst residues, as in the process describedimmediately above; solid polymer is then separated from the slurry,e.g., 'by filtration; and finally, the polymer is washed with a liquidhydrocarbon. This process, when properly carried out, results inextremely low levels of catalyst residues in the polymer. However, it isnecessary, in order to obtain the full benefits of the process, toeffect the separation of polymer in an inert atmosphere such asnitrogen, because contacting of the slurry with air and moisture causesprecipitation of the metal catalyst residues and consequently highermetal content in the 3,287,343 Patented Nov. 22, 1966 polymer. Themaintenance of an inert atmosphere during the separation of the polymeris cumbersome and costly. The art, therefore, would very much welcomeany improvement in the method which would permit handling thealcohol-treated slurry in the presence of air.

The present invention is directed to an improvement in the process ofthe aforesaid British patent, which improvement comprises adding to thealcohol-treated slurry from about 0.5 to about 1.5 moles of an alkanoicacid containing from 3 to about 22 carbon atoms per mole of total metalresidues in the slurry. By virtue of this improvement, the slurry issurprisingly rendered insensitive to air and moisture, and further stepsin the recovery and purification of the polymer can be taken withoutregard to the maintenance of an inert atmosphere. In fact, even when theslurry is allowed to come in contact with air of exceptionally highhumidity, it has nevertheless been possible on some trials to preparepolymers containing as little as 10 parts per million of total metal.

In accordance with an optional feature of the invention, there is addedto the slurry, either prior to or following the oddition of alkanoicacid, a small amount of alkylene oxide, such as propylene oxide, thepurpose of which is to neutralize any hydrochloric acid present in theslurry and thereby reduce its corrosivity to the apparatus employed inthe work-up. The addition of alkylene dioxide is, of course, notessential, but is desirable since it makes possible the use of lessexpensive ap paratus.

In order to avoid any premature oxidation of polymer which might occurupon filtration of the slurry, especially at elevated temperatures, itmay sometimes be desirable to add a small amount of an antioxidant suchas 2,6-di-t-'butyl-p-cresol to the slurry prior to filtration. Usuallyan amount of from about 0.1 to 1% antioxidant, based on the weight ofpolymer, will sufiice.

The invention is illustrated by the following examples in which partsand percentages are by weight unless otherwise specified.

Examples In these examples, propylene was polymerized by passing gaseouspropylene into a liquid, saturated aliphatic hydrocarbon diluent havinga boiling range of 170 to 200 C. containing titanium trichloride in theamount of 5 mM. per liter and diethylaluminum chloride in the amount of10 mM. per liter of liquid. The polymerization was carried out for 6hours at 50 C. under 28 p.s.i.g. of propylene.

At the end of the polymerization there was present in the polymerizer aslurry of fine particles of stereoregular polypropylene in thepolymerization diluent, the polypropylene constituting about 35% of theslurry.

To the slurry there was added a predetermined amount of an alkanol underan atmosphere of nitrogen and the slurry stirred for approximately 2hours at C. There was then added to the slurry a predetermined amount ofan alkanoic acid. (In some cases, 10 minutes prior to the addition ofthe alkanoic acid, propylene oxide in the amount of 3 ml. per liter ofdiluent was added.)

A portion of the slurry was removed from the polymerizer, stirred in airfor 10 minutes, filtered, and the filter cake washed with n-heptane at70 C., and the n-heptane removed by drying. The portion of the slurryremaining in the polymerizer was filtered under nitrogenwithout exposureto air, and the filter cake washed in the same manner that the filtercake from the air-exposed slurry was washed. The following tablepresents the data obtained from several polymerizations carried out inthe manner described and also control polymerizations in which alkanoicacid was not added to the slurry.

TABLE Polymer Analysis (parts per million) Propylene Example Alcohol(Percent of Oxide, Acid,

No. slurry) mlJliter Alkanoic Acid mMJmM. N Air diluent (Ti-l-Al) Ti AlTi A1 A n-Butanol (1.5) None 18 45 180 200 B Isopropanol (1 0)- 3 .do.10 30 200 *170 O. n Butano 3 -.do 26 48 200 200 1 do. 0 Stearic- 2 6. 316 9. 3 20 2--- .do- 3 -d0 1 17 15 15 14 3. d0 3 d0 1 5 *8 4 Isopropanol3 i do 1 8 15 *10 *13 5 n-Butanol (l.5) 3 do 0. 62 24 3O *18 *24 3 d0 0.50 19 26 *14 "23 3 Coconut fatty acid 8 0.8 28 38 *25 *32 3 Tall oilfatty acid... b 1. 5 14 23 *12 3 do b 1.0 8 18 *6 *14 3 Pelargonic 1.110 13 *10 *20 3 2-ethyl hexanoic 1. 0 14 28 *24 *30 3 d0 0.5 15 44 *13*44 3 Heptanoic 1. 0 8 30 *9 *31 3 d O. 67 18 *21 *17 3 1. 0 10 45 87200 3 0. 95 15 20 *12 *20 3 0. 65 18 28 *20 3 1. 0 5 32 *6 *31 3 0. 5 529 *23 *38 3 1. 0 5 22 5 30 3 0. 5 5 12 6 29 =air of 90-95% relativehumidity.

h =ealculated on average molecular weight of 250.

=calculated on average molecular weight of 190.

==0.05 mM. 2,6dit-butyl-p-cresol per mM. (TH-Al) also added.

As seen from the data in the table, when the alkanoic 30 acid is omittedas in Control A, the portion of the slurry that is worked up in thepresence of air yields a polymer of unacceptable high metal content,although the portion of the slurry worked up under nitrogen givesacceptably low metal content. In Controls B and C, it is shown that theaddition of propylene oxide has no eifect on reducing the metal contentof the polymer worked up in the presence of air; it does, however,reduce the acidity of the slurry and renders it less corrosive to metalapparatus. In Examples 1 to 14 and 16 to 21 it is seen that the additionof various alkanoic acids of 3 to about 22 carbon atoms in varyingamounts results in polymers having low ash contents, even when thework-up is carried out in the presence of air, and even when the air isof very high relative humidity. Example 15 shows the relativeineffectiveness of acetic acid whose number of carbon atoms is belowthree.

The process of the invention may be applied to the polymer produced inthe polymerization of any l-olefin, e.g., ethylene, propylene, butene-l,3-methylpentene-l, etc., or copolymers of such olefins, using theaforesaid catalyst system. In accordance with this known polymerizationmethod, the olefin is contacted at relatively low pressure andtemperature with a catalyst prepared by mixing a compound of atransition metal of Groups IV-B or V-B of the Periodic Table with anorganometallic compound of a metal of Group IILA of the Periodic Table.The so-called transition metal compound may be an inorganic salt such asa halide, oxyhalide, etc., or an organic salt or complex such as anacetylacetone, etc. Exemplary of the transition metal compounds that maybe used are titanium and zirconium tetrachlorides, titanium diandtrichloride, tetrabutyl titanate, zirconium acetylacetonate, Vanadiumoxyacetylacetonate, etc. The organometallic compound that is reactedwith one of the transition metal compounds or mixtures thereof may be,for example, triethylaluminum, tripropylaluminum, triisobutylaluminum,trioctylaluminum, tridodecylaluminum, dimethylaluminum chloride,diethylaluminum bromide, diethylaluminum chloride, ethylalurninumdichloride, the equimolar mixture of the latter two known as aluminumsesquichloride, dipropylaluminum fluoride, diisobutylaluminum fluoride,diethylaluminum hydride, ethylaluminum dihydride, diisobutylaluminumhydride, etc., and complexes, as for example, sodium aluminumtetraethyl, lithium aluminum tetraoctyl, etc.

Another method of carrying out the polymerization process is to use atwo-component catalyst system. In one such system the insolubleprecipitate which is formed by mixing the transition metal compound andthe organometallic compound as described above is separated and thenused in combination with an additional organometallic compound. Theinsoluble reaction product will be readily separated, if the reactiontook place in an inert diluent, from the diluent and soluble reactionby-products by centrifuging, filtering, or any other desired means. Insome cases it may be desirable to wash the insoluble reaction productwith additional amounts of hydrocarbon diluent in order to completelyremove all of the soluble by-products. This hydrocarbon-insolublereaction product is then used in combination with an organometalliccompound as exemplified above. This second catalyst component may be thesame organometallic compound that was used in preparing the insolublereaction product catalyst component or it may be a diflferentorganometallic compound. Of particular importance is the use of such ahydrocarbon-insoluble reaction product in combination with an aluminumtrialkyl such as triethylaluminum, triisobutylaluminum,trioctylaluminum, etc.

In another two-component catalyst system, the whole reaction mixtureformed on mixing a transition metal compound and an organometalliccompound may be used in combination with an additional organometalliccom pound, if the latter is halogen-free. This two component catalystsystem is particularly useful for the polymerization of linearl-olefins.

These polymerization processes are carried out in a wide variety ofways, as, for example, as batch or continuous operations and with orwithout the use of an inert organic diluent as the reaction medium.However, for the purpose of the present invention there must be used aninert, organic diluent that is liquid under the reaction conditions.Preferred diluents are saturated aliphatic hydrocarbons boiling in therange of 70'-250 C.

As pointed out already, the transition metal compound and theorganometallic compound may be reacted in situ, as, for example, in theparticularly effective method of polymerizing diolefins wherein atrialkylaluminum is reacted in situ with a tetraalkyl titanate. They mayalso be reacted prior to the introduction of the olefin or they may bereacted and then used in combination with additional increments duringthe polymerization and many other such variations may be utilized. Manyother variations may be made in the polymerization system to which theprocess of this invention may be applied. For example, when lowermolecular weight polymers are desired, a viscosity reducing agent suchas a haloalkane, as, for instance, carbon tetrachloride, etc., orhydrogen, or other such agent may be added.

According to the invention, the first step in working up the slurry torecover high quality polymer is treatment with an alkanol, the purposeof this step being to solubilize the catalyst residues and thus renderthem more easily removable in subsequent steps The alkanol used in thisstep is preferably one of those in the methanol to dodecanol seriesincluding branched chain alkanols, e.g., ethanol, propanol, isopropanol,n-butanol, sec-butanol, t-butanol, isobutanol, n-octanol,2-ethylhexanol, n-decanol, etc.

The amount of alkanol added at th end of the polymerization may bevaried widely and will depend chiefly upon the amount of catalyst usedin the polymerization and other reaction conditions, but in general isfrom about 0.1% to about 10% of the volume of the polymer slurry, andpreferably is from about 1% to about 5%. Much larger quantities may beused but are not believed to serve any useful purpose and hence are notusually desired for practical considerations. The alkanol treatment maybe carried out at any desired or practical temperature, as may be seenfrom the above examples, but generally will be from about C. to about100 C., and preferably from about 25 C. to about 80 C. Only a shortholding time is required for the alkanol treatment, depending upon thetemperature, catalyst, etc. In general, a period of from about minutesto 1 hour is adequate, but any longer period of time is practical, as,for example, overnight, etc. The alkanol treatment is carried out in aninert atmosphere, i.e., in the absence of oxygen, water, etc., and hencean oxygenand Waterfree atmosphere is used, as, for example, nitrogen orother inert gas.

The process as it has been described up to this point is well known inthe art, At this point, however, the invention departs from the art bythe addition of a small quantity of an alkanoic acid having from 3 toabout 22 carbon atoms to the alkanol-treated slurry. The acid can beadded While the slurry is at a temperature ranging from about 25 to 80C. The amount of acid can vary from about 0.5 to about 1.5 mole per moleof total metal in the slurry except that when the acid used containsonly 3 to 4 carbon atoms, a minimum of about 1 mole per mole of totalmetal is desirable. Although 1.5 mole of acid per mole of metal can beused, this is unnecessary and merely adds to the cost of the process.

The slurry is next simply agitated for a period of time ranging fromabout 5 to 60 minutes, and by virtue of this treatment has acquiredrelative insensitivity to air and moisture while the catalyst residuesremain in solution. Typical alkanoic acids that can be used in theprocess of the invention include both branched and straight-chain acids,e.g., propionic, n-butanoic, 2-ethylhexanoic, lauric acid, tridecoicacid, myristic acid, palmitic acid, stearic acid, behenic acid, and thelike.

It is now possible to separate the polymer from the slurry in thepresence of air, even very humid air. Separation can be eflected byfiltration, centrifugation, or any equivalent technique. The eliminationof the need for carrying out the separation in the absence of air is aremarkable step forward in the art since it eliminates thecumbersomeness and expense of maintaining an inert atmosphere.

Following separation of the polymer, it is simply washed with an inertliquid hydrocarbon. Any liquid hydrocarbon or mixture of suchhydrocarbons may be used for the washing, as, for example, aliphatichydrocarbons such as n-hexane, n-heptane, and aromatic hydrocarbons suchas benzene, toluene, and xylene, and cycloaliphatic hydrocarbons such ascyclopentane, cyclohexane, and the like. For practical reasons, thewashing is usually carried out at room temperature but may be carriedout at elevated temperatures up to about C., if desired.

Obviously, many variations may be made in the purification process ofthis invention. One such variation, which has been illustrated in theexamples, comprises the addition of propylene oxide in the amount ofabout 2 to 4 ml. per liter of diluent just prior to addition of thealkanoic acid. This addition of propylene oxide neutralizes any HCl thatis present in the slurry and makes it possible to carry out subsequentsteps in metallic apparatus that would otherwise be corroded.

In order to remove the hydrocarbon washing liquid from the hydrocarbon,a simple drying operation is adequate in most cases. In other cases, itmay be desirable to subject the polymer to a steam distillationtreatment before drying in order to remove all traces of the hydrocarbonliquid.

What I claim and desire to protect by Letters Patent is:

1. In the process for removing catalyst residues from a polyolefin whichhas been obtained by polymerizing an olefin in the presence of acatalyst comprising a compound of a metal selected from the groupconsisting of the metals of Groups IV-B and V-B of the Periodic Table incombination with an organometallic compound of a metal of Group III-A ofthe Periodic Table in the presence of an inert liquid diluent whereinthe slurry of solid polymer in diluent that is obtained from thepolymerization is treated with an alkanol to solubilize the catalystresidues, following which the polymer is separated from the slurry andwashed with an inert liquid hydrocarbon, the improvement which comprisesadding to the alkanol-treated slurry prior to separation of the polymeran alkanoic acid having from 3 to about 22 carbon atoms in the amount ofabout 0.5 to about 1.5 moles per mole of total metal in the slurry torender the slurry insensitive to air and moisture.

2. The process of claim 1 in which the polyolefin is polypropylene.

3. The process of claim 1 in which the polyolefin is polyethylene.

4. The process of claim 1 in which the alkanoic acid is stearic acid.

5. The process of claim 1 in which the slurry is pretreated withpropylene oxide to neutralize any hydrogen chloride present in theslurry.

No references cited.

JOSEPH L. SCHOFER, Primary Examiner. L. EDELMAN, Assistant Examiner,

1. IN THE PROCESS FOR REMOVING CATALYST RESIDUES FROM A POLYOLEFIN WHICH HAS BEEN OBTAINED BY POLYMERIZING AN OLEFIN IN THE PRESENCE OF A CATALYST COMPRISING A COMPOUND OF A METAL SELECTED FROM THE GROUP CONSISTING OF THE METALS OF GROUPS IV-B AND V-B OF THE PERIODIC TABLE IN COMBINATION WITH AN ORGANOMETALLIC COMPOUND OF A METAL OF GROUP III-A OF THE PERIODIC TABLE IN THE PRESENCE OF AN INERT LIQUID DILUENT WHEREIN THE SLURRY OF SOLID POLYMER IN DILUENT THAT IS OBTAINED FROM THE POLYMERIZATION IS TREATED WITH AN ALKANOL TO SOLUBILIZE THE CATALYST RESIDUES, FOLLOWING WHICH THE POLYMER IS SEPARATED FROM THE SLURRY AND WASHED WITH AN INERT LIQUID HYDROCARBON, THE IMPROVEMENT WHICH COMPRISES ADDING TO THE ALKANOL-TREATED SLURRY PRIOR TO SEPARATION OF THE POLYMER AN ALKANOIC ACID HAVING FROM 3 TO ABOUT 22 CARBON ATOMS IN THE AMOUNT OF ABOUT 0.5 TO ABOUT 1.5 MOLES PER MOLS OF TOTAL METAL IN THE SLURRY TO RENDER THE SLURRY INSENSITIVE TO AIR AND MOISTURE. 